1. Field of the Invention
This invention relates to the adaptation of lamps and associated lampholders for use in damp environments, and in particular to means for protecting such lamps and lampholders against flashover damage.
2. Description of the Prior Art
U.S. Pat. No. 4080030, which is hereby incorporated by reference, describes an arrangement for sealing a space in a lampholder in which electrical terminals of an installed lamp make connection with contacts of the lampholder. The sealing arrangement includes a resilient O-ring which is interference fitted in an undercut groove formed in a face of the lampholder aligned opposite a sealing surface of the lamp. In commercial use this arrangement has been found to have superior sealing reliability with respect to prior art arrangements. Nevertheless, in certain circumstances flashover damage has occurred to the lampholder. Customers have blamed this damage on improper sealing of the space where the lamp terminals make connection with the lampholder contacts.
An analysis of damaged lampholders returned by customers was conducted to determine the cause of the damage. The results of this analysis can best be understood by referring to FIG. 1, which shows a typical lampholder-mounting arrangement used by the customers experiencing the problem. In this mounting arrangement a
r fluorescent lampholder having a body 10 of insulating material is mounted in a hole in a horizontally-oriented panel 12 so that a fluorescent lamp can be installed vertically in an outdoor housing such as a lighted business sign.
The lampholder includes electrical contacts 14R, 14L disposed in a space 16 sealed by the resilient O-ring 18. The contacts are secured in the lampholder by respective integrally-formed terminals 20R, 20L which extend through the body 10 into a space 22 for the connection of leads 24R, 24L. The lampholder is secured in the panel 12 by means of metal spring clips 26R, 26L which are attached to the insulating body 10 by respective rivets 28R, 28L. The fluorescent lamp (of which only one end is shown) includes a sealed glass envelope 30 having at each end a concave portion fused around lead-in wires 32R, 32L. These lead-in wires are connected at their upper ends to a filament/electrode 34 and at their lower ends to respective lamp contacts 36R, 36L which are secured in an end insulator 38 of the lamp for making electrical connection with the contacts 14R, 14L of the lampholder. The end insulator 38 is secured in a metal end cap 40 which is affixed to the end of the glass envelope 30 by a cement 42.
The above description and FIG. 1 accurately depict the analyzed lamp-lampholder arrangement in all but two respects. First, the lampholder contacts 14R, 14L and the mating lamp contacts 36R, 36L have been rotated ninety degrees (around longitudinal axis 44) from their actual positions relative to the rivets 28R, 28L. This rotation has been effected to facilitate showing all of the relevant elements in one cross sectional drawing figure, but does not detract from an understanding of the analysis. Second, the means for securing the lamp contacts 36R, 36L in the end insulator 38 is not shown. There are many different well-known configurations of securing means used by lamp manufacturers; it would be neither fair nor useful to single out one specific configuration as more likely to be involved in the flashover problem under study, without evidence to that effect. Such evidence was not found.
The analysis of damaged lampholders repeatedly showed heavy damage in a concave region delineated by the broken line 45. This damage included total evaporation of the insulating body 10, of the head of rivet 28R, and of O-ring 18 in this concave region. Other damage which occurred repeatedly included blackening of the contacts 14R, 14L and of the rivet 28L. Customers who had examined the damaged lampholders before returning them blamed the damage on the leakage of rainwater or condensate around the O-ring and into the space 16. The customers deduced that water had run across the face of the lampholder body 10 filling the space 16 and recesses containing the heads of the rivets 28R, 28L, thereby forming a conductive path which shorted together the rivet heads and the two contacts 14R, 14L which provide electrical power to the lamp. The destructive capability of such a short circuit is evident in view of the fact that the rivets are electrically connected via the clips 26R, 26L to the panel 12, which typically is made from metal and connected to ground. However, during tests in which applicants subjected undamaged lampholders of the same type to a water spray (with a lamp installed but without the application of electrical power), no leakage by the O-ring was observed.